Medical implant with current leakage circuitry

ABSTRACT

Medical device implants for stimulating the nervous system of a recipient are disclosed. Embodiments include a cochlear implant with electrodes for delivering charge to the cochlea of the recipient and stimulation circuitry for delivering the charge to the electrodes. The medical devices include a transfer line which carries power and/or communication signals, the transfer line being in contact with tissue of the recipient when the medical device implant is implanted. A leakage capture conductor and/or sensing electrode is located or locatable proximate the insulated conductor.

BACKGROUND Field of the Invention

The present invention relates to the field of medical implants forstimulating the nervous system of a recipient, the medical implantsincluding implantable components and a power and/or signal transfer linebetween implantable components.

One form of medical implant for stimulating the nervous system of arecipient is a cochlear implant. A cochlear implant includes an implantcomponent and an array of electrodes. The implant component is designedto deliver a stimulating signal over a transfer line to the electrodes.The transfer line is typically a cable with a number of insulatedconductors corresponding to the number of electrodes in the array. Theelectrodes deliver charge of the stimulating signal to the cochlea, toexcite the auditory nerves of the recipient of the cochlear implant.

To evoke a hearing-like sensation with a particular apparent loudness ofintensity, the cochlear implant must deliver a time-constrained, minimumlevel of electric charge. A known issue with cochlear implants is therequirement that the net charge delivered to each electrode remain closeto zero in order to prevent non-reversible galvanic reactions, electrodecorrosion and the creation of chemical species injurious to adjacentcells and body tissue. Various methods have been proposed to addressthis problem, including the use of direct current blocking capacitorsconnected in series with the electrodes or particular switchingarrangements, which may for example periodically short circuitelectrodes to dissipate any accumulated charge.

A cochlear implant may include other potential sources of electriccharge that may result in a risk of adverse affects and/or reducedperformance. Any power and/or signal transfer line between componentparts of the cochlear implant may be a source of leakage current, forexample if the insulation around the transfer line fails or ifinsulation around a connection of the transfer line fails. In addition,for neural stimulation, it may be important that the signal provided tothe electrodes is clear and any irregular stimulation signals areminimised or eliminated.

SUMMARY

The invention generally relates to medical device implants forstimulating the nervous system of a recipient. Some of the disclosedembodiments include a cochlear implant with electrodes for deliveringcharge to the cochlea of the recipient and stimulation circuitry fordelivering the charge to the electrodes.

The medical devices include a transfer line which carries power and/orcommunication signals, the transfer line being in contact with tissue ofthe recipient when the medical device implant is implanted. A leakagecapture conductor and/or sensing electrode is located or locatableproximate the insulated conductor. In some embodiments the leakagecapture conductor and sensing electrode are distinct, and in others theyare the same component.

The leakage capture conductor is connected to a low impedance voltagereference, so as to carry leakage current from the transfer line to thelow impedance voltage reference, as an alternative to the leakagecurrent being carried in the recipient's tissue. In a cochlear implant,the voltage provided by the voltage reference may be selected to bebetween a maximum and minimum voltage applied to the cochlea by theelectrodes.

A switching circuit for delivering charge to the electrodes may bearranged so that when an electrode is connected to either the firstpower supply rail or the second power supply rail, current sources maybe interposed between the power supply rails and the electrode, wherebythe current sources provide high impedance connections. The provision ofa high impedance source for the electrodes may reduce distortion in thestimulation signal provided by the implant in the event that there isleakage current from the transfer line.

In some embodiments, the leakage capture conductor is located outsidethe transfer line so as to be in electrical contact with said tissue ofthe recipient when the medical device implant is implanted.

In some embodiments, the transfer line may carry a power signal from apower module housing a battery to a stimulation module housing thestimulation circuitry. The power signal may be an alternating currentsignal. Direct current blocking capacitors may be provided in serieswith the transfer line. In other embodiments, the leakage captureconductor is embedded in the insulation of the insulated conductor so asto be insulated from surrounding tissue when the medical device implantis implanted in a recipient. In these other embodiments, the leakagecapture conductor may be switched between a function of deliveringleakage current to the low impedance voltage reference and a function ofdelivering leakage current to a current detector.

The sensing electrode may be embedded in insulation of the transfer lineand connected to a current detection circuit, which detects currentbetween a point of failure of said insulation and an electrode. Theelectrode may be an electrode for stimulating the nervous system of therecipient or a diagnostic electrode connected to a switching circuit fortesting for the presence of leakage current.

Further embodiments will become apparent from the following description,given by way of example and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, preferred embodiments of the invention are nowdescribed with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic representation of a cochlear implant includinga leakage capture conductor in accordance with an embodiment of thepresent invention;

FIG. 2 shows a schematic representation of a cochlear implant includingan a leakage capture conductor in accordance with another embodiment ofthe present invention;

FIG. 3 shows a circuit diagram of a cochlear implant including a leakagecapture conductor in accordance with an embodiment of the presentinvention;

FIG. 4 shows a circuit diagram of a cochlear implant including a leakagecurrent detection circuit in accordance with an embodiment of thepresent invention;

FIG. 5 shows a circuit diagram of a cochlear implant including a leakagecurrent detection circuit in accordance with another embodiment of thepresent invention;

FIG. 6 shows a diagrammatic representation a cochlear implant includinga leakage current detection circuit in accordance with anotherembodiment of the present invention;

FIG. 7 shows a switching arrangement for providing both leakage currentdetection and leakage current capture functions in accordance with anembodiment of the present invention; and

FIG. 8 shows a flow chart of operation of a cochlear implant controllerincluding a leakage capture conductor and a leakage current detectioncircuit in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

The following description is given with reference to various embodimentsof the present invention. These various embodiments are described inconnection with a cochlear implant, however, embodiments of the presentinvention, such as those providing a path for leakage current and/ordetecting leakage current, may be applied to other medical implantdevices that include a power and/or signal transfer line.

FIG. 1 shows a diagrammatic representation of an embodiment of acochlear implant 100. The cochlear implant 100 includes a power sourcemodule 101 and a stimulator module 120.

The power source module 101 includes a battery 102 and an inverter 103for providing at a signal output 104 an AC power signal. The inverter103 may for example be an integrated circuit including an H-bridgeswitch configuration. The signal output 104 includes DC blockingcapacitors 105.

The stimulator module 120 includes a signal input 106, for receiving theAC power signal from the power source module 101. In some embodimentsthe signal input 106 may also be used as a signal input/output, thesignal for example being generated by modulating/coding a signal ontothe AC carrier. The signal input 106 includes two DC blocking capacitors107. A rectifier 109 is connected to the signal input 106 for convertingthe AC power signal into a DC supply signal comprising a positivevoltage signal V_(DD) and a negative voltage signal V_(SS), providedover supply rails.

The positive voltage signal V_(DD) and a negative voltage signal V_(SS)are received by a switching circuit 110, which controls the supply ofcurrent to and from stimulation electrodes ICE₁₋₂₂ and a referenceelectrode ECE₁. The number of stimulation electrodes and referenceelectrodes is a matter of design choice and may vary betweenimplementations. For example, as smaller electrodes and/or conductorsfor the transfer lines to the electrodes are used, then the number ofstimulation electrodes may increase. In the embodiment shown in FIG. 1,a second reference electrode ECE₂ is provided, which may provideredundancy for the reference electrode ECE₁ and/or may be used as asensing electrode, for example to provide for neural response telemetry.

The stimulator module 120 includes a controller 111. References to thecontroller 111 include both implementations as a single integratedcircuit microcontroller and a plurality of microcontrollers, each withassigned functions and with optional redundancy. One function of thecontroller 111 is to implement a control function to receive signals ata pickup coil 112 and, in response control the switching circuit 110 toprovide current to the stimulation electrodes ICE₁₋₂₂ and the referenceelectrode ECE₁ as required.

The AC power signal is communicated from the signal output 104 to thesignal input 106 over a transfer line in the form of an insulated cable113. The insulated cable 113 includes a pair of wires 114, 115 forcarrying the AC power signal and a reference voltage and insulation 116about the wires 114, 115. A leakage capture conductor 117 is provided soas to be located or locatable proximate the insulated cable 113 so as tobe in electrical contact with tissue of the recipient of the implant.For example, the leakage capture conductor 117 may be a wire coiledaround the outer surface of the insulated cable 113, optionally bondedto the insulated cable 113 through an adhesive, tie or otherwise. Inother embodiments the leakage capture conductor 117 may be a meshlocated around the insulation 116. More generally, the leakage captureconductor is any conductor positioned or positionable near the cablebetween the potential source of leakage current and the electrodes ICE,ECE.

In other embodiments the leakage capture conductor 117 may be a wire,mesh or other structure of conductive material embedded within theinsulation 116. When the leakage capture conductor 117 is within theinsulation 116, the insulation extends between the leakage captureconductor 117 and the wires 114, 115 to provide adequate insulation soas to not adversely affect the communication of the AC power signalduring normal operation. Embedding the leakage capture conductor 117 mayallow it to be used for more than one purpose, as explained below.However, it is expected that the performance of the leakage captureconductor 117 to capture leakage current may be less than optimum whenit is not in electrical contact with the surrounding tissue of therecipient of the implant.

The leakage capture conductor 117 may, for example, be made of platinum.Other suitable conductive materials may be used, having regard to theconductivity of the material, strength of the material and otherfactors, such as ease and cost of manufacture.

FIG. 2 shows a schematic representation of another embodiment of acochlear implant 200. The cochlear implant 200 has a similar structureto the cochlear implant 100, including the same power source module 101,insulated cable 113 and leakage capture conductor 117. The stimulatormodule 220 performs a like function to the stimulator module 120described above, but implements a bipolar stimulation strategy, notinvolving a separate reference electrode, when providing the stimulationsignals.

FIG. 3 shows a circuit diagram of an embodiment of circuitry 200 of thecochlear implant 100 shown in FIG. 1 and like reference numerals havebeen used for like components between FIGS. 1 and 3. The power sourcemodule 101 provides an AC signal having a DC voltage component regulatedby the voltage of the surrounding tissue. A reference electrode 201(which was not shown in FIG. 1) for providing a reference voltage forthe AC signal is provided for implantation in the tissue of therecipient, the reference electrode 201 being connected to the wire 114of the insulated cable 113 (the insulation 116 is not shown in FIG. 3).The AC signal is received by the rectifier 109, which provides a DCsignal output across a smoothing capacitor 202, the DC signal consistingof V_(DD) and V_(SS) provided to the switching circuit 110 on supplyrails, for convenience also referred to as V_(DD) and V_(SS).

The switching circuit 110 includes a plurality of current sources 203connected to V_(DD) and a plurality of current sources connected toV_(SS), so that the current sources 203 and 204 can operate in apush/pull arrangement when providing stimulation signals either betweenpairs of the stimulation electrodes ICI₁₋₂₂ (bipolar stimulation) orbetween a stimulation electrode ICI₁₋₂₂ and the reference electrode ECE₁or the reference electrode ECE₂ (monopolar stimulation). The stimulationsignals are commenced and ended by switches 205 operated according to aswitching strategy. The switching strategy is a design consideration andas mentioned herein above a typical switching strategy is for thecontroller 111 (not shown in FIG. 3) to control the switches 205 toprovide a biphasic pulse to the electrodes. Examples of other types ofpulses that form an alternating current signal and which may be providedby the switches 205 include triphasic pulses or asymmetrical pulses. Thestimulation electrodes ICI₁₋₂₂ are connected between the currentsources, as is the reference electrode ECE₁.

A voltage divider 206 is provided between V_(DD) and V_(SS). In theembodiment shown in FIG. 3 the voltage divider is provided by two largeresistors R1, R2. The voltage divider provides a DC reference voltageV_(ref). The magnitude of the reference voltage V_(ref) is dependent onthe stimulation strategy used, but a reference voltage V_(ref)=V_(DD)/2is V_(SS)=0 or (V_(DD)−V_(SS)/2+V_(SS)) if V_(SS)≠0 may suit a typicalsymmetrical biphasic pulse stimulation signal. If the stimulationstrategy is a triphasic pulse or other stimulation strategy thatprovides an asymmetrical pulse, then a suitable reference voltageV_(ref) is the mid point between the peak voltages provided on theforward and reverse current pulses, or if there are a plurality offorward current pulses and/or a plurality of reverse current pulses, ata mid point between the maximum voltage pulses.

The output V_(ref) from the voltage divider 206 is provided to a lowimpedance node 207. In the example shown in FIG. 3, the low impedancenode 207 is a voltage buffer. The combination of the voltage divider 206and the low impedance node 207 thus forms a low impedance voltagereference. The low impedance node 207 is connected to the leakagecapture conductor 117. In the event that the insulation 116 fails, theleakage capture conductor 117 and low impedance node 207 provide arelatively low impedance path for leakage current from the wire 115 whenthe AC power signal is high. Accordingly, the leakage current from thewire 115 will be received by the low impedance node 207, which acts as alow impedance current sink, rather than being conducted through tissueto the stimulation electrode ICI₁₋₂₂ that is connected to V_(SS) via aswitch 205 and current source 204. When the AC power signal on the wire115 is low, then the low impedance node 207 acts as a low impedancecurrent source, forming the primary path for current to the wire 115,reducing or substantially eliminating current to the wire 115 throughthe tissue.

The provision of the low impedance node 207 may both reduce the flow ofcurrent through tissue and also reduce distortion in the stimulationsignal due to leakage current that would otherwise flow between theinsulated cable 113 with failed insulation 116 and the electrodes.Accordingly, if the insulation 116 is breached, for example duringsurgical implantation or following surgical implantation, the cochlearimplant 100 may continue to operate effectively.

In alternative embodiments, the voltage divider 206 and low impedancenode 207 may be replaced by another low impedance voltage reference. Forexample, the low impedance node 207 may be omitted and R2 of FIG. 3 maybe replaced with a Zener diode, having a reverse breakdown voltage equalto V_(ref) discussed above. Alternatively, the low impedance node 207may be retained, with the zener diode providing the voltage referenceinput to the low impedance node 207. In still other embodiments, aband-gap voltage reference may be used in place of the voltage divider206 and low impedance node 207.

The current sources 203 and 204 each provide high impedance between thestimulation electrodes ICE₁₋₂₂ and reference electrode ECE₁ and thesupply rails V_(SS) and V_(DD). This provides an advantage that if theinsulation 116 fails, then the interference with the stimulation signalis minimized If using a single current source, for example, theswitching circuit arrangement shown in FIGS. 5 to 7, then there may beincreased interference with the stimulation signal in the event of theinsulation 116 failing. Another way to provide high impedance betweenthe stimulation electrodes ICE₁₋₂₂ and reference electrode ECE₁ and thesupply rails V_(SS) and V_(DD) is to provide a ‘floating’ currentsource.

For the implant 200 shown in FIG. 2 a similar circuit as shown in FIG. 3may be used, but without the ECE₁.

In some embodiments the leakage capture conductor 217 may perform thedual functions of operating as a reference electrode ECE and as aleakage capture conductor. For example, the leakage capture conductormay function as the reference electrode ECE₁ or the reference electrodeECE₂ described above with reference to FIG. 1. In these embodiments theleakage capture conductor 217 is provided in electrical communicationwith tissue, providing a current path from the stimulation electrodesICI₁₋₂₂ and the leakage capture conductor. In one implementation ofthese embodiments, an additional electrode ECE_(2A) may be connected tothe shield and low impedance node 207, allowing increased flexibility inthe physical location of the reference electrode.

While the foregoing description has described the leakage captureconductor 117 as being located to capture leakage current from a cablecarrying an AC power signal between implantable modules of a cochlearimplant, it will be appreciated that the leakage capture conductor 117may be located at other locations. For example, the leakage captureconductor 117 may be located about a transfer line in the form of aninsulated electrode cable 118 (see FIGS. 1 and 2) that carriesstimulation signals from the stimulator module 120, 220 to thestimulation electrodes ICI₁₋₂₂. In these embodiments, while leakagecurrent will be captured, in the event of an insulation breach theoperation of the cochlear implant to provide stimulation signals will beaffected. In addition, there may be a plurality of leakage captureconductors 117, provided for different parts of the cochlear implant,each connected to the same low voltage node 207 or a plurality ofleakage capture conductors may be distributed across a plurality of lowvoltage nodes. A single part, for example the insulated cable 113 mayhave one or more associated leakage capture conductors 117.

In addition or as an alternative to capturing leakage current,embodiments of the present invention (e.g., cochlear implants or othermedical implants) include an ability to detect leakage current thatoccurs due to a failure of insulation.

FIG. 4 shows an embodiment of an implant component 300 of a cochlearimplant including a current leakage detection function. Only thefunctional parts of the implant component 300 directly relevant toembodiment of the present invention illustrated in FIG. 4 are shown. Theimplant component 300 includes an implantable stimulator 300A and animplantable electrode array 300B. The implantable stimulator 300Aincludes a switching circuit 301. In the embodiment shown, the switchingcircuit 301 has a single current source 302, connected to a negativepower supply rail V_(SS). In other embodiments, the switching circuit301 may incorporate a push-pull current source configuration, forexample of the form described herein above with reference to FIG. 3 or afloating current source. These other embodiments may have particularapplication to when leakage current capture through a low voltage nodeas described herein above is used in combination with the ability todetect leakage current.

The switching circuit 301 includes a pair of switches S_(ICE(1-n)+),S_(ICE(1-n)−) for each stimulation electrode ICE₁ to ICE_(n) and a pairof switches S_(ECE+), S_(ECE−) for a reference electrode ECE₂. A furtherreference electrode (not shown in FIG. 4) to perform the function of theelectrode ECE₁ shown in FIG. 1 may be provided, particularly ifmonopolar stimulation is used. In addition, a further pair of switchesS_(A+), S_(A−) is provided between V_(DD) and V_(SS), connected to asensing electrode 304 through a capacitor 305 and a resistor 310. Forexample, the capacitor may have a capacitance of 100 nF and the resistora value of 100 kΩ.

An insulated wire 306 connects each stimulation electrode ICE_(1-n) tobetween a pair of switches S_(ICE(1-n)+), S_(ICE(1-n)−). Insulation 308is provided about each insulated wire 306. A controller, like thecontroller 111 (FIG. 1), operates the switches 301 and the currentsource 302 to achieve a required stimulation signal to the tissue. Forbipolar stimulation this is done through the stimulation electrodesICE_(1-n). For monopolar stimulation electrodes ICE_(1-n) are usedtogether with ECE₁ and/or ECE₂. A connection point 309 of the cableformed by the wires 306 and insulation 308 may be an insulated connectoror the cable may extend into the housing of the implantable stimulator300A.

The sensing electrode 304 is provided within the insulation 308. Thesensing electrode 304 is therefore normally electrically insulated fromthe tissue, from the wires 306 and from the stimulation electrodesICE_(1-n). The sensing electrode 304 may be a conductor extending alongand within the insulation 308, for example a wire in a coil shape, amesh extending along the insulated wires 306 or another configuration ofconductor extending along and at least over a portion of the insulation308 where insulation breaches are to be detected. The electrode 304 maybe platinum, or another suitable electrically conducting material.

During operation of the implant component 300 to provide signals to thestimulation electrodes ICE_(1-n), the switches S_(A+) and S_(A−) areboth open, so that the sensing electrode 304 is disconnected from V_(DD)and the current source 302. The operation of the switching circuit 301to use the sensing electrode 304 may be performed when the implantcomponent 300 is not providing stimulation signals, for example during adiagnostic phase, which may be performed during power on of the implantcomponent 300 and/or at other times.

With regard to FIG. 4, the first of two test phases will be described.In this phase at least one of the electrodes ICE_(1-n) is connected toV_(DD) by a switch S_(ICE(1-n)+) and the switch S_(A−) is connected toV_(SS) (via the current source 302). The switches S_(ICE(1-n)−) areopen. If the insulation 308 is in tact, then there will be a smallamount of current between the electrode(s) ICE_(1-n) connected to V_(DD)and the sensing electrode 304 due to capacitive coupling. If on theother hand the insulation 308 has been breached, there will be arelatively low impedance current path through the tissue between theelectrode(s) ICE_(1-n) connected to V_(DD) and the sensing electrode304. The resulting comparatively large leakage current can be readilydetected distinguished from the current due to capacitive coupling. Inthe embodiment shown, the leakage current flows from the electrode(s)ICE_(1-n) connected to V_(DD) to V_(SS) via the sensing electrode 304,the capacitor 305, the resistor 310 and the switch S_(A−). This currentmay be detected by a voltage detector 311 across the resistor 310. Thevoltage detector 311 may be a comparator that outputs a logical highwhen the voltage difference across the resistor exceeds a thresholdamount. In other embodiments, the voltage detector 311 may additionallyprovide an output indicative of a measurement of the magnitude of theleakage current.

In the second of two test phases, at least one of the electrodesICE_(1-n) is connected to V_(SS) by a switch S_(ICE(1-n)−) and theswitch S_(A+) is closed to connect the sensing electrode 304 to V_(DD)(via the capacitor 305 and the resistor 310). The switches S_(ICE(1-n)+)are open. In this test phase, current from the sensing electrode 304flows to the electrode(s) ICE_(1-n) connected to V_(SS). The voltagedetector 311 may be an integrator or comparator to detect both phases,or alternatively may detect current through only one of the phases—thetwo phases provided to reduce net DC charge being applied to theelectrodes over if only a single phase was used. In other embodiments,the test may involve a triphasic switching arrangement or otherasymmetrical switching arrangement (this may optionally match the normalswitching strategy for the stimulation electrodes). In still otherembodiments, when it is acceptable to deliver a net charge to theelectrodes, the test may involve just one of the test phases or anotherunbalanced switching strategy during the test.

After the test phases have been completed, the switches S_(A+), S_(A−)open, disconnecting the sensing electrode 304. The implant component 300then commences/recommences its functions to provide assisted hearing.

A sensing electrode may be provided in the insulation of a wire to areference electrode, for example ECE₁ shown in FIG. 1, either inaddition to or instead of the sensing electrode 304. If both areprovided this additional sensing electrode may have its own equivalentto switches S_(A+) and S_(A−), or may share the switches S_(A+) andS_(A−) with the sensing electrode 304. A distinction between these twoconfigurations is that in the earlier of the two breaches of insulationfor the ground electrode and stimulation electrodes can be detectedseparately. Similarly, a sensing electrode may be provided on othertransfer lines, for example the transfer line in the form of the cable113 shown in FIG. 1, either in addition to or instead of the sensingelectrode 304 and/or a sensing electrode for ECE₁.

If a breach in insulation is detected by the voltage detector 311, thenaction may be taken in response. For this purpose an output U of thevoltage detector 311 may be connected to the controller 111. The actionsmay include the controller 111 ceasing operation of the implantcomponent 300 and/or the controller 111 setting a flag, which iscommunicated to an external component that interrogates the implantcomponent 300 to indicate that a breach was detected.

FIG. 5 shows another embodiment of an implant component 400 of acochlear implant including a current leakage detection function. Likewith FIG. 4, only the functional parts of the implant component 400directly relevant to the embodiment of the present invention illustratedin FIG. 5 are shown. The implant component includes a switching circuit401, which includes a pair of switches S_(ICE(1-n)+), S_(ICE(1-n)−) foreach stimulation electrode ICE₁ to ICE_(D) and a pair of switchesS_(ECE+), S_(ECE−) for a reference electrode ECE₂. A further referenceelectrode (not shown in FIG. 5) to perform the function of the electrodeECE₁ shown in FIG. 1 may be provided, particularly if monopolarstimulation is used. A difference to the embodiment shown in FIG. 4 isthat the switching circuit 401 is part of a push-pull current sourceconfiguration and therefore includes multiple current sources. Theimplant component includes current sources 402A connected to V_(SS) andcurrent sources 402B connected to V_(DD). The controller of the implantcomponent (e.g. the controller 111 of FIG. 1) switches S_(ICE(1-n)+),S_(ICE(1-n)−) according to a switching strategy (e.g. biphasic ortriphasic pulses, monopolar or bipoloar) to provide the stimulationsignals to the electrodes ICE₁ to ICE_(n).

A leakage current detection circuit includes a capacitor 405 and aresistor 410, which may have the same values as the capacitor 305 andresistor 310. A voltage detector circuit 403 is provided across theresistor 410. To implement a test phase, the switch S_(A) is in a closedstate simultaneously with one or more of the switches S_(ICE(1-n)+). Fora single phase test, the resistor 410 may be connected to a voltagereference V_(ref) that equals V_(SS). In other embodiments, V_(ref) maybe at a mid-point between V_(SS) and V_(DD), enabling a second phase ofthe test, in which S_(A) remains closed while one or more of theswitches S_(ICE(1-n)−) is closed. The mid-point of V_(ref) may bedetermined the same way as V_(ref) shown in FIG. 3 for the low impedancenode 207. An output U indicates the presence of leakage current and thisoutput may be utilised for various actions, for example those describedabove with reference to FIG. 3.

FIG. 6 shows another configuration of a cochlear implant with a leakagecurrent detection circuit. The cochlear implant 500 includes a powermodule 501 and a stimulator module 502, in a like arrangement to thatdescribed above with reference to FIGS. 1 and 2. In this example acurrent detection circuit 503, which may be of the form of the switchesS_(A+) and S_(A−) and voltage detector circuit 311 or switch S_(A+) andvoltage detector circuit 403 described above in FIGS. 4 and 5respectively, is provided in the power module 501. The DC power supplyto the current detection circuit 503 may be sourced from the battery102, either directly or via a power supply circuit. For this embodimentthe leakage current detection circuit 503 detects failure in insulationof the power transfer line 504, carrying an AC power signal and/or amodulated communication signal. It achieves this by selectivelyconnecting an electrode 504, which is embedded in the tissue, to thepositive or negative power terminal of the battery or power supplycircuit and measuring current through the sensing electrode 505.

A cochlear implant may be configured to both capture leakage current anddetect failure of insulation. Both functions may be provided for thesame transfer line.

In some embodiments, the leakage capture conductor 117 is provided aswell as the sensing electrode 304. For example, for the same transferline, the leakage capture conductor 117 may be provided outside of theinsulation of the transfer line and the sensing electrode 304 or sensingelectrode 404 provided in the insulation.

In other embodiments, as illustrated in FIG. 7, where it is acceptablefor the leakage capture conductor 117 to be embedded in insulation, theleakage capture conductor 117 and the sensing electrode 304/sensingelectrode 404 may be the same component—leakage captureconductor/sensing electrode 601. In these embodiments, the conductor maybe switched between the low impedance node and the leakage currentdetection circuit. A switch 600, which operates under the control of amicrocontroller 602 (which may be the controller 111 of FIG. 1 withfunctionality to control the switch 600), to selectively connect aleakage capture conductor/sensing electrode 601 to a low impedance node,for example of the type described above with reference to FIG. 3 or to aleakage current detector, for example of the types described above withreference to FIGS. 4 and 5.

In some embodiments, the microcontroller 602 may connect the leakagecapture conductor/sensing electrode 601 to the leakage current detectioncircuit on initialisation of the implant and/or at periodic times forself-diagnosis cycles. At other times, the microcontroller may connectthe leakage capture conductor/sensing electrode 601 to the low impedancenode.

FIG. 8 shows a flow chart of a procedure that may be implemented by thecontroller 111 (see FIG. 1) when both leakage current detection andleakage current capture is provided. In step 1000, the controller 111 isinitialised, for example during power-on. A leakage current detectionstep 1001 is completed by switching the switch 600 to the leakagecurrent detection circuit and by controlled operation of the switchesS_(A) and S_(ICE). The result of the leakage current detection isevaluated (step 1002).

If leakage current above a threshold is detected, the threshold beingselected to be at a level higher than expected current due to capacitivecoupling, then responsive action is taken in step 1003. This may includepowering down the implant, setting a flag to indicate the detection ofleakage current for communication to a remote device upon interrogationor another action. Optionally the implant can be designed to continueoperating despite the detection of leakage current, in which case theprocess proceeds to step 1004. Otherwise, the process ends.

If leakage current above the threshold is not detected, the processproceeds from step 1002 to step 1004, with the controller 111 switchingthe switch 600 to the low impedance node. The switch 600 may remain inthis configuration until a diagnosis event occurs (step 1005) and thecontroller 111 may perform its functions to controllably sendstimulation signals to the cochlea via the electrodes ICE during thisstep. When a diagnosis event occurs, for example the elapsing of aperiod of time, a certain number of stimulation pulses or other metric,then the process returns to step 1001. If on a first diagnosis eventleakage current is detected and on a second diagnosis event leakagecurrent is not detected, then the process may optionally includeclearing any flag set in step 1003 between steps 1002 and 1004.

It will be appreciated that a method of producing a medical deviceimplant according to one embodiment of the present invention includes:

providing the medical device with a low impedance node electricallyconnected to a leakage capture conductor, wherein the low impedance nodeis configured to maintain the leakage capture conductor at anintermediate voltage between power supply rails of the medical deviceimplant; and

configuring the leakage capture conductor to be locatable proximate theexposed insulated conductor.

A controller of the medical device implant may be configured toselectively connect the power supply rails to the electrodes to providea symmetrical biphasic pulse and the method comprises configuring thelow impedance node so that the intermediate voltage is at a mid pointbetween voltages of the power supply rails.

Configuring the leakage capture conductor may comprise providing theleakage capture conductor so that it can be placed in electrical contactwith tissue of a recipient when the medical device implant is implantedin the recipient. The controller may provide a monopolar stimulationsignal to said electrodes and the method further comprises configuringthe medical device so that the leakage capture conductor functions as areference electrode for the monopolar stimulation signal.

A battery powered AC power supply may be provided for the medical deviceimplant in a first housing and the power supply rails in a secondhousing. The method may include connecting said exposed insulatedconductor to communicate power from the AC power supply between thefirst housing and the second housing.

It will also be appreciated from the foregoing description that a methodof producing a medical device implant of a type that is configured to,under the control of a controller, selectively connect power supplyrails to electrodes, includes, according to one embodiment of theinvention:

providing, within the insulation of an insulated conductor for carryingelectrical charge between component parts of the medical device asensing electrode, the sensing electrode electrically insulated fromboth the conductors of the insulated conductor and the and theenvironment outside the insulation; and

providing a connection for said sensing electrode to a current detectioncircuit operable to detect current between a point of failure of saidinsulation and an additional electrode, wherein the current detectioncircuit is configured to provide an output indicative of detection ofsaid current.

The additional electrode may be an electrode that is selectivelyconnected to power supply rails under the control of the controller.

The method may further include configuring the controller to:

selectively connect each of the electrodes to a first power supply railor a second power supply rail;

selectively connect the sensing electrode to the second power supplyrail when said additional electrode is connected to the first powersupply rail; and

selectively connect the sensing electrode to the first power supply railwhen said additional electrode is connected to the second power supplyrail.

The method may further include configuring the controller to:

selectively connect each of the electrodes to a first power supply railor a second power supply rail;

connect the sensing electrode to a voltage reference node at a voltagebetween voltages if the first and second power supply rails when saidadditional electrode is connected to the first power supply rail; and

connect the sensing electrode to a voltage reference node at a voltagebetween voltages if the first and second power supply rails when saidadditional electrode is connected to the second power supply rail.

It will be understood that the invention disclosed and defined in thisspecification extends to all alternative combinations of two or more ofthe individual features mentioned or evident from the text or drawings.All of these different combinations constitute various alternativeembodiments of the invention.

1. A medical device implant comprising: stimulation circuitry, thestimulation circuitry comprising first and second power supply rails andarranged to provide first and second direct current voltages to thefirst and second power supply rails respectively and a controllableswitching circuit for, under the control of a controller, selectivelyconnecting the first and second power supply rails to electrodes; aninsulated conductor operable to communicate electrical charge, whereinthe medical device implant is arrangeable so that insulation of theinsulated conductor is in contact with tissue of a recipient when themedical device implant is implanted in the recipient; a leakage captureconductor locatable to capture leakage of the electrical charge from theinsulated conductor; and a low impedance node electrically connected tothe leakage capture conductor, wherein the low impedance node isoperable to maintain the leakage capture conductor at an intermediatevoltage between the first and second direct current voltages.
 2. Themedical device implant of claim 1, wherein the switching circuit isarranged so that when said electrode is connected to either the firstpower supply rail or the second power supply rail, the power supply railpresents a high impedance source to the electrode.
 3. The medical deviceimplant of claim 2, wherein the stimulation circuitry comprises a firstcurrent source electrically connected between the first power supplyrail and an output to an electrode and a second current sourceelectrically connected between the output and the second power supplyrail.
 4. The medical device implant of claim 1, wherein the controllercontrols the controllable switching circuit to provide to said electrodea symmetrical biphasic pulse and wherein the intermediate voltage is ata mid point between the first and second voltages.
 5. The medical deviceimplant of claim 1, wherein the leakage capture conductor is inelectrical contact with said tissue of the recipient when the medicaldevice implant is implanted in the recipient.
 6. The medical deviceimplant of claim 1, wherein the stimulation circuitry is containedwithin a first housing, the medical device implant comprises a secondhousing containing a power source for providing an alternating currentpower signal, and the insulated conductor comprises at least one leadconnected to carry the alternating current power signal from the powersource to the stimulation circuitry.
 7. The medical device implant ofclaim 6, comprising direct current blocking capacitors in series withthe insulated conductor.
 8. The medical device implant of claim 1,wherein the insulated conductor is a cable connecting the stimulationcircuitry to said electrodes.
 9. The medical device implant of claim 1,wherein the stimulation circuitry is configurable to provide a monopolarstimulation signal to said electrodes and is configured so that theleakage capture conductor functions as a reference electrode for themonopolar stimulation signal.
 10. The medical device implant of claim 1,wherein the leakage capture conductor is located in a substantiallyfixed location proximate the insulated conductor.
 11. The medical deviceimplant of claim 1, wherein the leakage capture conductor is embedded inthe insulation of the insulated conductor so as to be insulated fromsaid tissue when the medical device implant is implanted in therecipient.
 12. The medical device implant of claim 11, furthercomprising a controlled switching circuit configured to: selectivelyswitch the leakage capture conductor into and out of said electricalconnection with the low impedance node; and selectively switch theleakage capture conductor into electrical connection with a leakagecurrent detection circuit.
 13. The medical device implant of claim 12,wherein the controlled switching circuit is configured to switch theleakage capture conductor into electrical connection with the leakagecurrent detection circuit during a diagnostic cycle only and switch theleakage capture conductor into electrical connection with the lowvoltage node substantially at all other times.
 14. A medical deviceimplant comprising: stimulation circuitry for providing stimulationsignals to electrodes, an insulated conductor for carrying electricalcharge, wherein the medical device implant is structured so that whenthe medical device implant is implanted in a recipient, insulation ofthe insulated conductor is in contact with tissue of the recipient; asensing electrode in the insulation; and a current detection circuitelectrically connectable to the sensing electrode so as to detectcurrent between a point of failure of said insulation and an additionalelectrode, wherein the current detection circuit is configured toprovide an output indicative of detection of said current.
 15. Themedical device implant of claim 14, wherein said additional electrode isconnected to the stimulation circuitry.
 16. The medical device implantof claim 15, wherein the stimulation circuitry comprises first andsecond power supply rails for providing first and second direct currentvoltages respectively and controllable switches to: selectively connecteach of the electrodes to the first power supply rail or the secondpower supply rail; connect the sensing electrode to the second powersupply rail when said additional electrode is connected to the firstpower supply rail; and connect the sensing electrode to the first powersupply rail when said additional electrode is connected to the secondpower supply rail.
 17. The medical device implant of claim 15, whereinthe stimulation circuitry comprises first and second power supply railsfor providing first and second direct current voltages respectively andcontrollable switches to: selectively connect each of the electrodes tothe first power supply rail or the second power supply rail via acurrent source; connect the sensing electrode to a voltage referencenode at a voltage between the first and second direct current voltageswhen said additional electrode is connected to the first power supplyrail; and connect the sensing electrode to a voltage reference node at avoltage between the first and second direct current voltages when saidadditional electrode is connected to the second power supply rail. 18.The medical device implant of claim 17, wherein the stimulationcircuitry is contained within a first housing, the medical deviceimplant comprises a second housing containing a power source comprisinga battery and configured to provide an alternating current power signal,and the insulated conductor is configured to carry the alternatingcurrent power signal from the power source to the stimulation circuitryand wherein the medical device implant is configured to selectivelyconnect said additional electrode to the power source to provide acurrent flow path through tissue of a recipient of the medical devicebetween the additional electrode and the sensing electrode if theinsulation is breached.
 19. The medical device implant of claim 13,wherein the sensing electrode comprises a wire or mesh extending alongsubstantially the entire length of the insulated conductor.
 20. Themedical device implant of claim 13, wherein the current detectioncircuit provides an output indicative of a magnitude of detectedcurrent.
 21. A method of operating a medical device implant, the methodcomprising: selectively providing high and low voltages to electricallystimulate a recipient of the medical device implant; maintaining a lowimpedance electrical path at an intermediate voltage between the highand low voltages; and capturing current leaked from the medical deviceimplant during execution of said selectively providing via the lowimpedance electrical path.
 22. The method of claim 21, wherein asymmetrical biphasic pulse is used to electrically stimulate therecipient and wherein the intermediate voltage is at a mid point betweenthe high and low voltages.
 23. The method of claim 21, wherein saidcapturing current comprises using a conductor that is in electricalcontact with tissue of the recipient of the medical device implant, theconductor positioned between a source of said current leaked from themedical device implant and the location of said selectively providing.24. The method of claim 23, wherein a monopolar stimulation signal isused to electrically stimulate the recipient, wherein said capturingcurrent comprises using an electrode to capture said current andadditionally using the electrode as a reference electrode for themonopolar stimulation signal.
 25. A method of operating a medical deviceimplant in a recipient, the method comprising: communicating anelectrical signal over a transfer path of the medical device implant,wherein the transfer path is normally insulated from tissue of therecipient; maintaining a sensing electrode about said transfer path, thesensing electrode normally insulated from said tissue and said transferpath; and detecting current from said power transfer path to saidsensing electrode due to failure of insulation there between; andperforming an action in response to said detecting.
 26. The method ofclaim 25, comprising selectively connecting the sensing electrode topower supply rails of the medical device.
 27. The method of claim 25,further comprising: selectively connecting each of a plurality ofadditional electrodes for providing stimulation signals to the recipientto a first power supply rail or a second power supply rail; selectivelyconnecting the sensing electrode to the second power supply rail or thefirst power supply rail when a said additional electrode is connected tothe first power supply rail or the second power supply railrespectively.
 28. The method of claim 25, further comprising:selectively connecting each of a plurality of additional electrodes to afirst power supply rail or a second power supply rail; connecting thesensing electrode to a voltage reference node at a voltage betweenvoltages of the first and second power supply rails when a saidadditional electrode is connected to the first power supply rail; andconnecting the sensing electrode to a voltage reference node at avoltage between voltages of the first and second power supply rails whena said additional electrode is connected to the second power supplyrail.